In a typical form of process for the production of a hollow body of thermoplastic material by means of blow molding, a preform or parison is accommodated in a blow molding mold comprising at least first and second mold portions, and expanded therein under the effect of an increased internal pressure within the preform or parison. The mold portions are moved with a reciprocating motion between an open condition in which they are at a spacing from each other and a closed condition in which they bear against each other and with their mutually facing sides define a mold cavity whose contour corresponds to the configuration of the preform when expanded by the increased pressure therein. The movement of the mold portions is produced by at least one drive system for moving them between the open and closed conditions.
In regard to the closing and opening movements of the mold portions, it is generally necessary for those movements to take place in a defined fashion relative to a reference plane of the main frame structure of the blow molding machine, and in such a way that such movements can be satisfactorily reproduced at any time. The usual procedure in that respect is that the mold portions move with a symmetrical motion towards the central plane of a preform which is disposed centrally between them, and then move, again with a symmetrical motion, away from the hollow body which is produced from the preform by expansion thereof. The preform generally hangs freely between the mold portions, before they are moved together.
Accordingly, the central plane of the preform is primarily referred to hereinafter as the above-mentioned reference plane. It should be appreciated however that other planes may constitute the reference plane, if so required.
Particularly when suing a hydraulic drive system for producing the closing and opening movements of the mold portions, by virtue of the characteristics of the drive means used in such a system, it is not possible, without taking additional precautionary steps, to achieve the desired synchronism in respect of the mold portions in the closing and opening movements thereof relative to the preform, so that there is the risk that, due to different speeds of movement the mold portions do not reach their respective open or closed condition in the proper definable positions. Such irregularities in the movements of the mold portions are generally undesirable, especially as they can occur in an uncontrollable fashion and can thus differ from one operating cycle of the machine to another.
In order to avoid such irregularities in the movements of the mold portions, as an additional precautionary arrangement in that respect the machine may have synchronisation devices which cause the mold portions to move with a defined motion relative to a reference plane of the main frame structure of the machine. The arrangement is generally such that the synchronisation assembly has at least one mechanical connecting element which is arranged non-displaceably on the main frame structure of the blow molding machine and which kinematically couples the mold portions together. The fixed arrangement of the connecting element on the main frame structure of the machine provides that the closing and opening movements of the mold portions occur in the same fashion but in opposite relationship to each other, relative to the central plane of the preform or the article produced therefrom. That design configuration ensures, while using simple means, that the mold portions open and close symmetrically and thus move into their closed condition or their open condition in properly defined positions.
In one form of such a synchronisation arrangement, the connecting element is a gear which is disposed rotatably on a shaft which is fixedly mounted on the main frame structure of the blow molding machine. The two mold portions have toothed racks which each mesh with the gear. When one of the mold portions moves, the gear is rotated by the corresponding rack, with the result that the other mold portion whose associated rack is disposed parallel to the other rack on the other side of the above-mentioned shaft carrying the gear, is moved in the opposite direction.
In another design configuration of a synchronisation arrangement, a control lever is mounted on the main frame structure pivotably about a stationary shaft. Pivotably connected to the two lever arms of the control level, which are of the same length, are rods or bars which at their other end are each movably connected to a respective mold portion. The mold portions are kinematically positively connected together and to the main frame structure of the blow molding machine, by arrangements as just described above.
Other forms of synchronisation arrangement may also be used, which are generally familiar to any person skilled in this art and which for that reason do not require more detailed description herein. Such arrangements may comprise for example an endless flexible element such as a chain, cable or the like, which is passed around two wheels, rollers or the like which are mounted rotatably but non-displaceably on the frame structure of the machine. However a common aspect of all the above-indicated constructions is that they have at least one member which is arranged stationarily even if rotatably on the main frame structure of the blow molding machine, and thus establish a reference plane, relative to which the mold portions move symmetrically in opposite relationship to each other. The reference plane in such cases is generally so selected that it coincides with the central plane of the preform, as indicated hereinbefore. The arrangement is frequently also such that disposed in the above-mentioned central plane is the blowing pipe member which co-operates with the blowing mold and the preform and through which inter alia the pressure fluid required for expansion of the preform is passed into the blow molding mold.
A possible design configuration of such a blow molding machine is to be found for example in U.S. Pat. No. 4,150,080, the disclosure of which is hereby incorporated into this application in this respect.
Particularly in the last phase of the closing movement of the mold portions, a symmetrical arrangement of the mold portions in that way is frequently desirable as, in that phase, the two mold portions embrace the preform which is hanging from an extrusion head or other suitable holding means, and squeeze off the waste material present on the preform, while at the same time the portions or regions of the preform which bear against each other are welded together to produce a closed volume for expansion of the preform by the production of an increased internal pressure therein. In the majority of cases, when the preform occupies its receiving position in which it is received by the blow molding mold, the preform will be hanging from the extrusion head of an extrusion apparatus. As however the preform can also be produced in a different fashion from extrusion, it is also possible for the preform to be held in its receiving position by another form of holding means, until it has been received by the blow molding mold.
In the course of extending the range of use of the blow molding process and in particular also for the production of hollow bodies of a highly complicated configuration, it has been found that the above-described movements of the mold portions, which occur symmetrically relative to the preform, are not always advantageous, especially as the blow molding mold is not symmetrically divided in all cases, with the result that the mold portions of the blow molding mold are of mutually different configurations. There are therefore possible uses in which it may be more appropriate or even necessary for the movements of the blow molding mold portions to be such that they occur asymmetrically relative to the preform. That may also apply in regard to the opening movements of the mold portions, relative to the manufactured article which after expansion of the preform is generally held by the blowing pipe member or another member which co-operates with the blow molding mold and which does not participate in the opening movement of the mold portions.